The prevalence of driving while affected by cannabis is rising. It has been shown that drugs are detected commonly among those involved in motor vehicle accidents, various studies reporting that up to 25% of drivers involved in accidents tested positive for illicit drugs, with cannabis being the most common found, followed by benzodiazepines, cocaine, amphetamines and opioids. It is apparent that drugs, when taken in combination with alcohol, and multiple drugs, present an even greater risk; drug driving is a significant problem, both in terms of a general public health issue and as a specific concern for drug users.
The primary active component of cannabis is Δ9-tetrahydrocannabinol (THC), the structure of which is shown below:

Studies have repeatedly shown that THC impairs cognition, psychomotor function and actual driving performance. For example, it has been reported that the degree of performance impairment observed in experimental studies after doses up to 300 μg per kg of THC were equivalent to the impairing effect of a blood alcohol concentration at the legal limit for driving under the influence in most European countries. The combined use of THC and alcohol produces severe impairment of cognitive, psychomotor, and actual driving performance and increases the risk of crashing.
Cannabinoids (C21 compounds typical of and present in cannabis, their carboxylic acids, analogues, and transformation products) are routinely determined by gas chromatography-mass spectrometry (GC-MS). This approach requires complex instrumentation and all samples must be derivatized prior to injection. High-performance liquid chromatography, utilising electrochemical detection, has also been used. Low detection limits are achievable but high potentials are required for the electrochemical oxidation of cannabinoids. Typically, potentials of up to 1.2 V are required, which is close to the decomposition of water which increases the background current and introduces noise. Backofen et al (2000, BioMed. Chrom., 14:49) recently addressed this problem and explored non-aqueous electrolyte systems at platinum and gold electrodes, observing reduced noise and allowing a low detection limit of ca. 0.1 μM. This limit is two orders of magnitude lower than on-column UV detection and compares favourably with GC-MS.
As mentioned above, electrochemical methodologies have been employed as end of column detectors for THC. Typical sensing of THC is based on the oxidation of the hydroxyl group. This technique is not ideal since the electrochemical oxidation of phenols in aqueous solution is plagued by irreversible adsorption of oxidation reaction intermediates and products producing fouling of the electrode surface. This leads to poor electrode response and reproducibility, although this can be overcome to some extent by using low phenol concentrations and/or elevated temperatures. Alternative methods include the use of laser ablation to remove such passivating electrolytically generated layers or high overpotentials, which increase the anodic discharge of the solvent generating hydroxyl radicals which degrade the adsorbed oligomeric and polymeric products on the electrode surface.
A standard analytical technique for determining substituted phenol compounds is via reaction with the Gibbs reagent, i.e. 2,6-dichloro-p-benzoquinone 4-chloroimine. Gibbs showed that quinonechloroimides react with phenolic compounds producing brightly coloured indophenol compounds, which can be conveniently monitored via spectrophotometry. It was generally believed that the position para to the hydroxyl must be unsubstituted (Gibbs, (1927) J. Biol. Chem., 71:445; and Gibbs, (1927) J. Biol. Chem., 72:649). Gibbs reported that the pH of the solution greatly affects the rate of formation of the indophenol compound: at a pH of 10 the beginning of indophenol blue formation was observed to occur within two minutes, while at pH 8.5 this timescale was increased to 16 minutes. Dacre (1971, Anal Chem., 43:589) explored a large range of phenolic compounds and concluded that the Gibbs reaction was non-specific. A few substituted phenols were also reported as giving a negative Gibbs reaction.
Josephy and Damme (1984, Anal. Chem., 56:813) explored the Gibbs reaction with para-substituted phenols. The reaction mechanism is shown in Scheme 1 below:

The mechanism involves first the solvolysis of the Gibbs reagent (1) which yields dichloro-benzoquinone monoamine (2). This attacks the para position of the phenol resulting in an adduct (4) which deprotonates with the resulting intermediate (4) losing a proton and R−, the para-substituted leaving group, to form 2,6-dichloroindophenol (5). Note that in the case R═H, (4) is oxidised to (5) by reaction with a second molecule of (2). The resulting indophenol is brightly coloured and can be easily characterised via spectrophotometry. However in their work, Josephy and Damme noted several exceptions which did not give a positive Gibbs reaction. These included halogen-substituted phenols (TCP, TBP and TIP), hydroxybenzaldehydes and related compounds, hydroxybenzyl alcohols and hydroxybenzoic acids. The reason why was not elucidated.
Green tea (Camellia Sinensis) is a rich source of polyphenol compounds known as catechins. Catechins are effective anti-cancer and anti-tumour agents and are claimed to have anti-mutagenic, anti-diabetic, hypocholesterolemic, anti-bacterial and anti-inflammatory properties. The most abundant catechins are (−)-epigallocatechin gallate (EGCG) and (−)-epigallocatechin (ECG) the structures of which are shown below:

EGCG and EGC are thought to be the most effective catechin compounds, and the important characteristics of green tea, e.g. taste, nutritional values, palatability and pharmacological effects, depend substantially on their polyphenol content.
Methods of detecting catechins include high performance liquid chromatography using end of column detectors such as a coulometric array, UV, mass spectrometry and electrochemical detection. Caffeine, a major component in tea, can interfere with the UV analysis of catechins. Chromatographic methods coupled with electrochemical detection showed improved selectivity since caffeine is electrochemically inactive. Such a technique is based on simply holding an electrode at a suitably high potential which corresponds to the electrochemical oxidation of the analyte of interest. However, it is well documented that the electrochemical oxidation of phenolic compounds results in deactivation of the electrode surface (Pelillo et al, Food Chem. 87, (2004), 465; and Wang et al, J. Electroanal. Chem. 313, (1991), 129); a passivating polymeric film is produced which decreases the sensitivity and degrades the reproducibility although this can be overcome to a certain extent by using low phenol concentrations. The electrode materials employed in electrochemical end of column detectors include noble metals (Sano et al, Analyst 126, 2001, 816; and Yang et al, Anal. BioChem. 283, 2000, 77) and glassy carbon (Kumamoto et al, Anal. Sci., 16, 2000, 139; and Long et al, J. Chrom. B 763, 2001, 47) electrodes. Recently, Romani et al (J. Agric. Food Chem. 48, 2000, 1197) explored screen-printed electrodes modified with tyrosinase enzyme as an electrochemical end of column sensor where the disposable aspect overcomes electrode fouling and alleviates the need to polish the electrode surface between runs.
In summary, the methods described above are limited by the complexity of instrumentation, a need to derivatize samples, unacceptable detection limits, high oxidation potentials or a lack of specificity.